You would need 9.3 gallons of bleach to raise your chlorine from .5 to 20 ppms in a 28,000 gallon pool with 6% bleach.

You have to put 28,000 in the "ppol size"
6 in the "Chlorine strength"
20 in the "ppm" increase"

click on calculate, and it will show how many gallons in the shaded area on the bottom.